Update document action
Update document action

This commit adds the `update` action to the `DocumentsController`. Much
of this is similar to `create` so it should be easy to refactor it out.

alphagov/specialist-publisher-rebuild,alphagov/specialist-publisher-rebuild,alphagov/specialist-publisher,alphagov/specialist-publisher,alphagov/specialist-publisher,alphagov/specialist-publisher-rebuild,alphagov/specialist-publisher-rebuild